The present invention relates to a new construction for the face wall of a golf club head.
Nearly all modern, popular heads called "woods", such as the driver and the fairway woods, are in the form of hollow shells, usually of metal. Driver heads must not weigh more than about 210 grams, or there is an unacceptable penalty in maximum distance of drives. The present inventors have done research which indicates that for maximum drive distance, optimum head mass may be as small as 180 grams and the shaft may be longer than usual. This finding is in reasonable agreement with modern trends in driver design. In addition, a large face is highly desirable because it strongly reduces the percentage of hits which are partly off the face (which the present inventors call POF hits). The present inventors have found that large faces are especially important because these POF hits are usually the worst hits a golfer makes. Large moments of inertia of the club head about its center of gravity are also highly desirable because they reduce errors caused by hits which are somewhat off center. Large size correlates closely with large moments of inertia, because this puts mass farther from the center of gravity.
These considerations bring about a design limitation in the maximum size of face which will have adequate strength for withstanding impact of club head and ball. The present invention respects this limitation, while concurrently allowing club heads to have larger faces.
FIG. 1 (prior art) shows an elevation view of a common design of bridge trusses for illustrative purposes. Supports are indicated at numeral 10 at the ends. At mid-span, the truss is often deeper (thicker) than at the supports as indicated at 12 to accommodate the greater bending stresses in this region. This has limited similarity to the construction of the face wall in the present invention. Such configurations have not been used in connection with golf club faces in the years during which hollow club head construction has been favored. There are other important differences from a beam, such as the club face wall of the present invention being a continuous structure rather than an assembly of beams, the requirement for the ball hitting surface to be an integral part of the structural elements, and the face surface being elliptical in shape, or having other shapes which are used on golf clubs.
FIG. 2 (also prior art) is a downward looking cross section of the face wall of a typical modern prior art "wood" type club head which is made of metal. The face wall, which has a hitting surface 13, has small ribs 14, extending from top to bottom of the face wall, which are integrally formed and intended to improve the strength of the face wall without much increase in face wall mass. The present inventors have shown that the addition of small ribs such as those illustrated in FIG. 2 actually tend to reduce the strength of the face wall if the face wall mass is maintained constant.
U.S. Pat. No. 5,380,010 issued to the present inventors, and U.S. Pat. No. 5,464,211 (C. Atkins), U.S. Pat No. 5,570,886 (Rigal et al), U.S. Pat. No. 4,076,254 (G. Nygren), and U.S. Pat. No. 664,438 all show internal bracing between the inside surface of the face wall and other parts of the club head to provide adequate face strength. In order to maintain total head mass at a desired value, all involve removing peripheral mass and adding at least part of the removed mass at locations nearer to the center of gravity to provide the internal bracing, thus lowering moments of inertia.
U.S. Pat. No. 4,903,781 (D. Allen) shows a honeycomb structure to support the face. It has nominally uniform bending strength.
U.S. Pat. Re. No. 34,925 (J. McKeighen) shows a construction using a face wall which varies in thickness in an opposite sense, with thicker outer portions and a thinner center portion as compared with the present invention. As a result, it actually requires greater face wall mass for adequate strength.
U.S. Pat. No. 5,163,682 (G. Schmidt et al.) describes a face wall structure in which, compared with the center thickness, the face wall is thinner toward the toe or toward the heel, or both. Toward the toe end, it is of constant thickness in the up-down direction. Toward the heel end, there is a thickened region which starts approximately at the face center and runs toward the lower part of the heel end of the face, its purpose being to facilitate the flow of metal into the face wall when the head is cast. The present invention not only uses thickness variation in the toe-heel direction, but also in the up-down direction, and whereas patent '682 specifies the presence of the thickened portion running toward the heel, the present invention does not depend on any such thickened portion running toward the heel. U.S. Pat. Nos. 5,318,300 and 5,474,296, (both to Schmidt et al.) are similar in construction to each other.